1. Field of the Invention
The present invention relates to a motor driving device for driving a motor.
2. Description of the Prior Art
A motor driving device is incorporated, for example, in a disk recording/reproducing apparatus for recording data on and reproducing data from a disk-shaped recording medium. In such a motor driving device, to keep the rotation speed of a motor constant, the rotation rate of the motor is detected, and the amplitude of a motor driving signal is controlled in accordance with the difference signal (error signal) that represents the difference between the detected rotation rate and the specified rotation rate. Here, when an extremely large error occurs, there is a risk of an overcurrent flowing through the motor. To prevent this, a current limiter circuit is provided to restrict the current flowing through the motor within a certain limit. FIG. 4 shows the current limiter circuit employed in a conventional motor driving device. As shown in this figure, in this current limiter circuit, the output of a constant-current circuit CC is connected to the input of a current mirror circuit CM composed of an input-side NPN-type transistor Q1 and an output-side NPN-type transistor Q2.
The emitter of the transistor Q1 and the emitter of the transistor Q2 are connected through resistors R2 and R3, respectively, to a reference voltage GND. The collector of the output-side transistor Q2 of the current mirror circuit CM is connected through a resistor R1 to a supplied voltage V.sub.CC, with which a spindle motor is driven. The voltage at the node between the resistor R1 and the collector of the transistor Q2 is fed out as a current limit voltage V.sub.CL.
The error signal (error voltage) mentioned above is, together with the current limit voltage V.sub.CL output from the current limiter circuit, fed to a selector. The selector outputs the error voltage when the error voltage is higher than the current limit voltage and outputs the current limit voltage when the error voltage is lower than the current limit voltage. The output of this selector is fed through a current feedback amplifier to an amplitude control circuit for controlling the amplitude of the motor driving signal;.
However, configured as shown in FIG. 4, this current limiter circuit employed in a conventional spindle motor driving device as mentioned above has the following disadvantage. Suppose that the output current of the constant current circuit CC is I, the resistance of the resistor R1 is R, and the resistors R2 and R3 have equal resistances. Then, the current limit voltage V.sub.CL is given by EQU V.sub.CL =V.sub.CC -I.multidot.R
Here, since the resistance varies with temperature, the current limit voltage VCL is prone to be influenced by heat. That is, it is difficult to keep the value of I.multidot.R (=V.sub.CC -V.sub.CL) within.+-.several percent of 200 [mV] irrespective of temperature.
Accordingly, the limit of the driving current of the spindle motor is prone to vary with heat, and thus there is a risk of a current higher than the intended limit flowing through the spindle motor, causing thermal runaway. This often affects adversely the other circuits constituting the motor driving device, making the motor driving device unreliable against heat.